High-viscosity-index lubricating oils are used on a large scale for the formulation of multi-purpose lubricating oils. They can be prepared, for instance by peroxidation of linear paraffins having at least 10 and at most 19 carbon atoms per molecule (hereinafter referred to as "linear C.sub.10 -C.sub.19 paraffins"). The peroxidation should be carried out by contacting the paraffins at an elevated temperature with an organic peroxide of the general formula R-O-O-R.sup.1, wherein R and R.sup.1 represent alkyl, aryl or acyl moieties. Linear C.sub.10 -C.sub.19 paraffins, together with branched C.sub.10 -C.sub.19 paraffins are found in light mineral oil fractions, such as kerosine and gas oil fractions. The paraffins can be separated from said mineral oil fractions by cooling. From the paraffin mixtures obtained the desired linear paraffins can be isolated by way of fractional crystallization or complexing with urea. The linear C.sub.10 -C.sub.19 paraffins thus obtained are usually contaminaed with sulfur- and nitrogen compounds from the mineral oil, and also cyclic compounds. Preparatory to being suitable for use as feed for the preparation by peroxidation of high-viscosity-index lubricating oils, the C.sub.10 -C.sub.19 paraffins should be freed from these contaminants.
Linear C.sub.10 -C.sub.19 paraffins which can suitably be used as starting material for the preparation by peroxidation of high-viscosity-index lubricating oils may also be synthesized starting from a mixture of carbon monoxide and hydrogen. In this process (which is termed a Fischer-Tropsch synthesis) a H.sub.2 /CO mixture is contacted at elevated temperature and pressure with a catalyst comprising one or more metals from the iron group together with one or more promoters and a carrier material. The preparation of these catalysts can suitably be carried out by the known techniques, such as precipitation, impregnation, kneading and melting. As compared with waxy light mineral oil fractions, the products prepared by the Fischer-Tropsch synthesis have the advantage that they contain virtually no sulfur- and nitrogen compounds and cyclic compounds. Nevertheless there is a drawback to using the products obtained over the usual Fischer-Tropsch catalysts for the preparation of high-viscosity-index lubricating oils, which drawback is connected with their composition. For the C.sub.10 -C.sub.19 compounds are made up to a considerable extent of branched paraffins, branched and unbranched olefins, and oxygen-containing compounds.
Recently there has been found a class of Fischer-Tropsch catalysts which have the property of yielding a product wherein the C.sub.10 -C.sub.19 compounds consist virtually exclusively of linear paraffins. The Fischer-Tropsch catalysts belonging to the above-mentioned class contain silica, alumina or silica-alumina as carrier material, and cobalt together with zironium, titanium and/or chromium as catalytically active metals, in such quantities that per 100 parts by weight (pbw) of carrier material, the catalysts comprise 3-60 pbw of cobalt and 0.1-100 pbw of zirconium, titanium, and/or chromium. The catalysts are prepared by depositing the metals involved on the carrier material by kneading and/or impregnation. For further information concerning the preparation of these catalysts by kneading and/or impregnation reference may be made to Netherlands patent application No. 8301922, recently filed by the Applicant. Considering the composition of the product prepared over the cobalt catalysts it is extremely attractive to separate from said product a light fraction substantially consisting of C.sub.10 -C.sub.19 paraffins and to convert at least part of this light fraction by peroxidation into a product containing the desired high-viscosity-index lubricating oil.